The present invention relates to an offshore structure for use in an arctic environment and more particularly relates to an offshore structure for use in ice-infested areas which includes a mooring system which is capable of resisting the destructive forces caused by movement of the ice and at the same time, is relatively easy to maintain and service even when the surface is ice covered.
The increasing demand for petroleum products has required exploitation of many new regions throughout the world. One of the most promising of these regions is the arctic areas lying offshore of northern Alaska, Canada, and Greenland. However, obviously the exploitation of such areas present technological problems not normally encountered in the routine offshore exploitation for petroleum.
One obvious problem in exploiting offshore arctic areas lies in coping with the masses of ice which continuously form during certain parts of the year. These ice masses include sheets of ice having thicknesses of eight feet thick or more which may have "pressure ridges" formed therein which may measure 100 feet or more in thickness. These ice masses are not stationary and may move several hundred feet per day under the influence of surface winds and currents. Obviously, these moving ice masses develop substantial forces which, in turn, are normally destructive to any fixed objects lying in the path of the ice masses.
Accordingly, it has been recognized for sometime that bottom-supported structures of the type routinely used in ice free offshore areas are particularly vunerable in arctic offshore areas since the moving ice masses contact the fixed structures near the water line, thereby setting up bending moments which eventually crush or buckle the structure. Various configurations, e.g. an inverted conical platform section, have been proposed for bottom-supported structures which function to break and deflect these ice masses around the structure as the ice moves pass. Examples of such structures are shown in U.S. Pat. Nos. 3,952,527; 3,972,199; 4,075,964; and 4,103,199. However, as the depths of water in the arctic areas increase to 200 meters or more, the practicality of using any bottom supported structure decreases.
In providing arctic offshore structures for use in deep water, several structures have been proposed which in effect comprise a floating frusto-conical shaped or similarly-shaped hull which is either dynamically positioned or is moored in place by a catenary line anchor system; see U.S. Pat. Nos. 3,766,874; 3,837,311; 3,872,814; 3,902,447; 4,073,144.
A typical example of such a floating system is disclosed in U.S. Pat. No. 4,043,943 wherein a floating hull having an upper frusto-conical shape is moored in a deep, ice-infested area by means of a catenary mooring system. The mooring system is designed to permit active heaving of the hull in the water to thereby use the hull itself, to break the contacting ice. While this type structure appears capable of successfully operating in arctic areas, certain problems may arise in maintaining and servicing its mooring system during the ice forming seasons. That is, if an anchor on a catenary mooring line breaks loose while the surface is covered with thick sheets of ice or an anchor line, itself, breaks, there is no effective way to retrieve, replace, or reposition that anchor or anchor line since the use of an anchor handling vessel, necessary for such operations, is impractical if not impossible under these conditions. Therefore, if a floating hull is to be used in exploiting deep arctic offshore areas, a need exists for properly mooring the hull not only to provide the necessary stability for the hull but also provide for both routine and emergency maintainence of the mooring system even when the surface is covered with ice.